Field of the Invention
The present invention relates to an image capturing apparatus and a method of controlling the image capturing apparatus, and more specifically relates to technology for performing focus adjustment by an on-imaging plane phase difference detection method.
Description of the Related Art
In recent years, in an image capturing apparatus of which a single-lens reflex camera is representative, increasing weight is being given to a shooting method of shooting while viewing a live view (LV) screen. Various techniques have been proposed as autofocus (AF) methods in an image capturing apparatus, and as primary techniques there are a phase difference detection method and a contrast detection method.
In the phase difference detection method, light beams from a photographic subject that have passed through mutually differing exit pupil regions in an imaging optical system form an image on a pair of line sensors, and a defocus amount of the imaging optical system is calculated from a phase difference of a pair of image signals obtained from that pair of line sensors. Then, an in-focus state is obtained by moving a focusing lens by a movement amount that corresponds to the defocus amount that was calculated (see Japanese Patent Laid-Open No. 9-054242). However, with the above phase difference detection method, a light path to the image sensor is blocked, so shooting while viewing a LV screen is basically not possible.
In the contrast detection method, an in-focus state is obtained by searching for a focusing lens position where a contrast evaluation value generated from the image signal obtained using the image sensor becomes largest while moving the focusing lens (see Japanese Patent Laid-Open No. 2001-004914). The contrast detection method performs focusing based on the image signal, and therefore is appropriate for AF when performing LV shooting, and in recent years this method has become a mainstream AF method used when performing LV shooting. However, with the above contrast detection method it is not possible to easily determine the position and direction to move the focusing lens in order to focus on the subject. Therefore, with the contrast detection method, in some cases it takes time to perform focusing due to a mistake in the direction in which focusing should be performed or passing by the focus position.
Shooting while viewing an LV screen, for which there is increasing demand in recent years, is used not only for shooting still images but also for shooting moving images, so in the AF control, in addition to responsiveness for performing focus, quality of the focusing operation has been sought. Recently, AF methods have been proposed that are capable of focusing with high speed and high quality even when performing LV shooting. One example of those methods is an on-imaging plane phase difference detection method, in which the above phase difference detection method is performed on the plane of the image sensor.
As one type of on-imaging plane phase difference detection method, a technique has been proposed in which image forming pixels and focus detection pixels provided in some lines are arranged in an image sensor, and while performing image capturing with the image forming pixels, on-imaging plane phase difference detection is performed by comparing output signals of the focus detection pixels (see Japanese Patent Laid-Open No. 2000-156823). By using the on-imaging plane phase difference detection method, it is possible to perform autofocusing with a phase difference detection method even in LV shooting, so focusing can be performed with high speed and high quality.
Also, in recent years there is a shooting method in which in LV shooting, a subject such as a face is detected, and AF is performed on the detected face. In Japanese Patent Laid-Open No. 2013-003501, a technique is proposed in which two AF methods, both an on-imaging plane phase difference detection method and a contrast detection method, are provided, and which of the two AF methods to use is predicted and switched according to an arrangement relationship between the position of a detected subject and focus detection pixels of the on-imaging plane phase difference detection method.
However, when executing AF with the above on-imaging plane phase difference detection method, there are cases where although expected that focus point following could be quickly performed, due to movement of the subject the focus point could not be followed, so it takes some time until AF completes and as a result a photo opportunity is lost.